Infrared and ab initio study of the relative stability and geometry of the 3-fluoropropene-hydrogen chloride van der Waals complexes

The mid-infrared spectra (4000-400 cm(-1)) of complexes between the cis and gauche conformers of 3-fluoropropene, CH2=CHCH2F, and HCl have been recorded in liquefied argon at temperatures ranging from 103 to 123 K. From this data, the complexation enthalpy for the complex with cis-3-fluoropropene was determined to be -9.6(3) kJ mol(-1), while that for the complex with gauche 3-fluoropropene was estimated, from the observed HCl stretching frequency, to be -10.4(5) kJ mol(-1). The stoichiometry of these complexes was determined from constant temperature, variable concentration experiments to be 1:1. A few weak bands in the spectra are assigned to higher order complexes. An ab initio study of the structures and vibrational frequencies of the complex was performed at the B3LYP/G-311++G(d,p) level. These calculations show that HCl can form a van der Waals bond with either the carbon-carbon double bond or the fluorine atom. Comparison of the experimental frequencies with the calculated ones shows that all observed complex bands were due to complex formation with the fluorine atom.